The invention disclosed herein pertains generally to data transmission systems and more particularly to data interface methods and apparatus employed in serial data transmission systems.
The switching system described herein is a PCM time division multiplex system designed to accept analog voice information and convert it into corresponding digital representations for subsequent network switching and reconversion into the analog form. Such a PCM switching technology is explained in "A review of Telecommunications Switching Concepts--Part 1" and Part 2 thereof, published in Telecommunications February, 1976 and March, 1976 respectively.
Such systems allocate dedicated time paths for carrying digital words representative of corresponding analog voice samples. The majority of these systems are designed for switching analog voice information in the range of about 300 Hz to 4 KHz. To obtain an adequate sampling rate, such a PCM system samples a plurality of analog voice signals at a typical 8 KHz rate, i.e. every 125 microseconds. The samples are encoded into eight-bit digital words.
Early attempts at switching digital data information exclusively through the voice channels of such PCM systems were successful, but only at the expense of denying the use of those channels for PCM voice transmission. Without equipment to synchronize the transmitted digital data with the switching system sampling rate, only asynchronous data up to 4 kilobits per second (Kbs) could be sampled and switched accordingly.
While technical developments have led to the availability of synchronizing equipment capable of being interfaced with the voice ports for collecting and storing the data bits and supplying the same to the PCM system in synchronism with the system clock, such advancements have allowed the systems to switch data rates only in the order of about 8 Kbs or less, and yet with a sacrifice in voice channel capacity.
The majority of long distance data communications is achieved by way of telephone lines or other single wire mediums. Such established transmission facilities necessitate the use of a serial data transmission mode. For a given transmission rate, it is recognized that less information can be conveyed by serial data via a single bus as contrasted with parallel words transferred via a multi-wire parallel bus. However, present economics dictate that for distances beyond a few hundred feet the serial data transmission technique is yet the most advantageous.
Because of the widespread use of serial data transmision there have evolved certain transmission rates which have become somewhat standard. For instance, data transmission rates of 50, 75, 150 bits per second (bps) and doubles of 150 bps up to 9600 bps are presently considered as standard rates. The choice of a particular transmission rate depends in large part on the type of equipment utilized to transmit, receive or process the data. Teletypewriters, for example, can handle only very slow transmission rates while high speed computing systems may accommodate transmission rates in excess of 9600 bps.
With these diverse data transmission rates, and the accompanying modems and related equipment available today, there is a corresponding need to interface one serial data transmission rate with that of another rate. The telecommunication switching system, by way of example, operates at a typical base frequency or rate of 8000 cycles per second. Thus, each voice port or data port is serviced cyclically every 125 microseconds. Such a rate, being set and not one of the noted standard rates, must yet be dealt with to accommodate the various data streams.
It is, therefore, a primary object of the present invention to provide a data interface circuit for converting serial data streams of one transmission rate in corresponding serial data streams of another transmission rate.
It is another object of the present invention to provide a data interface circuit for synchronously converting a serial data stream of one transmission rate into a corresponding serial data stream of another rate.
It is yet another object of the invention to provide methods and apparatus for converting 9600 bps data streams, having data bits and start/stop format bits, into 8000 bps data stream in synchronism with such latter mentioned streams.
The foregoing as well as other objects and advantages will become apparent by referring to the detailed description of the preferred embodiment, which follows hereinafter, together with the appended drawings.